Acoustic microscope with a control and data capturing device

ABSTRACT

An acoustic microscope allowing both the topography and the elasticity of a sample to be measured at the same time. To this end the displacement of a cantilever with a tip is measured by the deflection of a laser beam. In order to measure the topography, the average deviation of the tip is held constant by a regulation circuit. The regulation circuit consists of a split-photodiode which supplies a neutral signal to the output of a normalizing amplifier which delivers a neutral value. Deviations from this neutral signal are compensated by a z-electrode of a piezocrystal. The elastic properties of the sample are measured by coupling ultrasound into the sample by means of a transducer and the high-frequency displacements of the cantilever with the tip are detected by a second detection device that consists of knife-edge detector and a fast photodiode. The detection device may also consist of a heterodyne time-of-flight interferometer or a capacitive detection scheme.

This application is a division of Ser. No. 545,849 filed Nov. 13, 1995now U.S. Pat. No. 5,675,075.

BACKGROUND OF THE INVENTION

The invention concerns an acoustic microscope for examination of aspecimen, with a nib affixed to an elastic beam and arranged in thenear-surface area of a specimen surface, with a supersonic transducercoupled to the specimen, with a device for positioning the specimenrelative to the nib, where the nib has at time average a consistentdistance from the surface, and with a control and data capturing device.

An acoustic microscope of this kind is known from WO 89/12805. On thisacoustic microscope, a nib attached to a tuning fork is arranged in thenear-surface area of a specimen surface. Forming an elastic beam, thetuning fork of piezoelectric quartz can be excited to mechanicalvibrations by application of an electrical alternating voltage viaelectrodes attached to its shanks. These mechanical vibrations couple byway of the nib, as ultrasound, in the surface and, depending on theinteraction between the vibrating nib and the specimen surface, lead onaccount of the attenuation of the vibration to a shift of the vibrationfrequency and/or the amplitude of the tuning fork as against a freevibration. The specimen and nib allow positioning relative to each otherby means of a moving apparatus, the nib being at time average arrangedat a consistent distance from the surface. A control and data capturingdevice serves the coordination of the data sensed by the nib incontingence on the relative position of the nib to the specimen.

In this acoustic microscope the tuning fork with the nib attached to itserves both as supersonic transmitter and also as sensor. Therefore, anelectronic substraction of substantially equal values for the frequencyand amplitude of the mechanical vibration must be carried out forobtaining the measured values, which subtraction is relatively prone toerror. The result, notably with short measuring times per measuringpoint on the surface, is an unfavorable, poor signal-to-noise ratio.Owing to the resonant stimulation of the tuning fork in sustainedoperation, a scan of the topography of the surface, separate of sensingfor instance elastic properties, can basically not be performed in thecoupling area between the nib and the surface. A further provision withthis acoustic microscope is stimulating the tuning fork preferably atits resonant frequency in the range of about 32 kHz, so that, while theamplitude is high in relation to the stimulating force, coupled-ininterference vibrations cause in this frequency range as wellexcessively high interference amplitudes, which unfavorably affect thesignal-to-noise ratio.

Another acoustic microscope is known from the publication "ScanningMicrodeformation Microscopy," by B. Cretin and F. Sthal in the magazine"Applied Physics Letters" 62 pp 829 through 831 (1993). In this device,the vibrations of the nib at approximately 50 kHz produce at theresonant frequency of the elastic beam microdeformations on the specimensurface which induce an acoustic wave in the specimen. The acoustic waveallows detection by amplitude and phase relative to the vibrations ofthe nib, with a supersonic transducer. Plotting the amplitude and/orphase of the supersonic wave allows the generation of an image of theelastic properties of the specimen surface scanned by the nib, incontingence on the position of the nib. Possible also is detecting andimaging material inhomogeneities contained beneath the surface. Thelocal resolution of this microscope ranges at around 10 μm.

With such an acoustic microscope it is possible to detect mechanicallyhard and soft areas of the specimen surface. But a topography of thespecimen surface is possible only indirectly, by processing thesupersonic signals received, and proves to be very difficult, notablywith relatively complex semiconductor topographies. With modernsemiconductor structures ranging in the order of a few μm, thismicroscope cannot be used for high-resolution examinations on suchspecimens.

A further disadvantage of this acoustic microscope is the relatively lowsignal-to-noise ratio. The vibrating nib acts as a point source, so thatthe amplitude of the ultrasound after passage through the specimen, inaddition to the attenuation in the medium of propagation, due to thespherical wave characteristic of the broadcast ultrasound, is very lowat the point of detection. Therefore, amplification of the signaldetected by the supersonic transducer is provided for, using a lock-inamplifier, in order to improve the signal-to-noise ratio. However, thisentails a relatively long measuring time.

The publication "Using Force Modulation to Image Surface Elasticitieswith the Atomic Force Microscope," by P. Maivald, H. J. Butt, S. A. C.Gould et al. in the magazine "Nanotechnology" 2, page 103 ff (1991),describes an atomic force microscope which allows measuring in localdependence the elasticity properties in the area of the specimensurface, in addition to the direct measurement of the topography of aspecimen while keeping the average force acting on the elastic beamconstant via an actuator element, by cyclically moving the specimen toand fro in relation to the nib. The elastic beam deflection is inmechanically softer areas greater than it is in mechanically harderareas. Thus, the topography as well as the elasticity properties in thenear-surface area can be imaged by plotting the maximum deflections ofthe nib at constant force in dependence on the position of the nib.

With such an atomic force microscope it is difficult to separate theshares which in the contrast stem from the elasticity and from thetopography. The modulation frequency for reciprocating the specimen isin the range of few kHz, in order for the phase shift betweenstimulation and deflection to be maximally low. In the described forcemicroscope, the repetition rate is 5 kHz, of which results, in additionto the risk of a superimposition by vibrations of the mechanicalstructure of the acoustic microscope, a relatively long measuring timefor the elasticity measurement.

The problem underlying the invention is to provide an acousticmicroscope which allows measuring both the topography and elasticity ofthe specimen, at a high local resolution and a high signal-to-noiseratio as well as contrast, quickly and independently of one another.

SUMMARY OF THE INVENTION

This problem is inventionally solved in that the supersonic transduceris a transmitting head with which at a distance from the tip or nib,ultrasound can be coupled into the specimen, where the ultrasound has afrequency which is higher than the resonant frequency of the elasticbeam or cantilever, with the nib attached to it, in that the elasticbeam, the moving apparatus, the transmitting head and the specimen arerigidly joined to one another, and in that the nib deflections caused bythe ultrasound coupled in can be sensed with a detection device at aconsistent distance, averaged over the deflections and kept constantwith the aid of a control circuit, between the nib and the surface ofthe specimen.

Sensing the nib deflections allows direct capture of the amplitude ofthe supersonically induced surface waves as a measuring signal for theelasticity measurement. By keeping a mean distance between the nib andthe surface of the specimen constant with the aid of a control circuitit is possible to image the surface topography with a coordinatedcontrol signal, independently of the supersonically induced deflectionsof the nib. A high signal-to-noise ratio and a high contrast areachieved by the mechanically rigid joining of the components and byproviding a higher supersonic frequency in relation to the resonantfrequency of the elastic beam with the nib attached to it.

A first detection unit with a slow response time is favorably used inthe control circuit for keeping the mean deflection of the nib constant,by means of a piezoelectric crystal. But it is also possible to obtain acontrol signal by low-frequency filtering from the signal of a singlewide-band detector unit, which signal comprises also the supersonicallyinduced high-frequency signal.

Ultrasound with a frequency of at least several MHz is preferably used,so that low-frequency interferences in the range of the resonantfrequency of the elastic beam with the nib attached to it can beseparated in a simple manner.

The elastic beam, positioning device and transmitting head are favorablyjoined to one another, mechanically rigidly, by means of a clamp orholder, where the resonant frequency of the holder is lower than thesupersonic frequency, thus guaranteeing a sufficient mechanicalstability of the acoustic microscope. Low-frequency interferences, suchas for instance mechanical vibrations in the structure, have nounfavorable effect, due to measuring the nib deflection far above theinherent frequency of the elastic beam with the nib attached to it.

In one embodiment of the invention, the nib deflections are sensed byway of a deflection of a light beam reflected on the elastic beam. As afirst optical detector unit, the control circuit features a bisectedphotodiode of an arrangement such that at a certain interactive forcebetween nib and specimen surface the light beam incidence is centeredbetween the two segments of the photodiode. The bandwidth of thebisected photodiode is considerably smaller than the frequency of theultrasound coupled in, so that the high-frequency deflections of the nibescape sensing by the first detector unit.

The detector device features a second optical detector unit, which inthis embodiment is formed by a single photodiode with a bandwidth of atleast the ultrasound frequency, and features a smooth-edged baffle, forinstance a razor blade, the two being so adjusted to one another thatthe baffle blocks at constant, average interactive force between nib andspecimen surface, in the absence of ultrasound, substantially one-halfof the light beam.

In a further embodiment, the second optical detector unit consists of aheterodyne travel time interferometer where the frequency of the lightin the long interferometer arm can be shifted by means of a frequencyshifting device by a predetermined amount and the two superposed outputbeams fall each on one photodiode. A difference demodulation amplifierallows forming the amplified difference signal of the two photodiodes,which subsequently, by demodulation of the shifting frequency, containsan output signal which matches the deflection of the nib, and thus ofthe elastic beam.

In another embodiment of the invention, the control circuit and thedetector device are integrated in a capacitive detector where themeasuring capacity is formed of the elastic beam and a needle-shapedelectrode arranged opposite from it. The high-frequency changes of themeasuring capacity can be separated, by a high-pass filter, from thepart of the measuring capacity change separated via a low-pass filterand used as control signal. An optical structure that is sometimessensitive to adjustment can be circumvented thereby.

Further embodiments and advantages of the invention derive from thesubclaims and the following description of figures. These show in

FIG. 1, a schematic illustration of an acoustic microscope with adetection device and a control circuit with two optical detection unitsfor sensing the topography and supersonically induced deflections of thenib;

FIG. 2, a block diagram of a transceiver unit for generation and pickupof the nib deflections;

FIG. 3, an acoustic microscope in schematic illustration, with aheterodyne travel time interferometer as a second optical detection unitfor sensing the supersonically induced nib deflections; and

FIG. 4, an acoustic microscope in schematic illustration, where acapacitive detection device is used to sense the supersonically induceddeflections of the elastic beam.

FIG. 1 shows schematically an acoustic microscope. A tip or nib 1 isattached with its base to one end of an elastic beam or cantilever 2 ofSi₃ N₄ having a length of approximately 100 μm. In this embodiment, thenib 1 has the shape of a pyramid and at the point a radius of curvatureof about 50 nm. The elastic beam 2 has a spring modulus of about 0.1Newton per meter. With its other end, the elastic beam 2 is arranged onthe upper end 3 of a mechanically rigid clamp or holder 4.

A tubular piezoelectric crystal of about 1 cm diameter and 1 mm wallthickness is applied on the base 5 of the holder 4. To performperpendicular movements, the piezoelectric crystal features in a planeabout its outer surface four electrodes which, offset 90 degrees, covereach one-fourth of the outside surface. The inside surface is coatedwith a grounded electrode. Two electrodes 6, 7 are schematically shownin FIG. 1. A rectangular movement in one plane can be performed byapplication of control voltages upon the two visible electrodes 6, 7offset by 90 degrees. The two not visible, peripherally appliedelectrodes are wired to the negative voltage of the respective oppositeelectrode. One of the visible electrodes 6, 7--along with the pertaininginvisible electrode offset by 180 degrees--serves to perform themovement in the x-direction and is referred to hereafter as x-electrode6. Electrode 7, along with the appropriately wired opposite electrode,serves the movement in the y-direction perpendicular to the x-directionand is referred to hereafter as y-electrode 7.

One end of the tubular piezoelectric element is provided with an annularz-electrode 8, which encloses the entire circumference and serves theperformance of a movement perpendicular to the plane of motion ofx-electrode 6 and y-electrode 7. For better presentation, the drawingshows the size ratios distorted.

Situated on the z-electrode 8 is a transmitting head 9 with whichultrasound can be coupled in a specimen 11 through a intermediate body10. The nib 1 rests on a surface of the specimen 11. But it may also bespaced a few hundred nanometers from the surface of the specimen 11.

The electrodes 6, 7, 8 connect each via pertaining piezoelectric controllines 12, 13, 14, a high-voltage amplifier 15 and a digital-analogconverter 16 to a control and data capturing device 17. The ultrasonictransducer includes transmitting head 9 which connects via a line 18 toa transceiver 19, by means of which, as will be described in more detailfarther down with reference to FIG. 2, the deflection of the nib 1 bythe ultrasound can be measured. The deflections of the nib 1 amount toabout 10 nanometers in the present embodiment.

Focussed by a lens 21, a laser beam 22 from a semiconductor laser diode20 emitting at a wavelength of about 670 nm falls on the flattened freeend of the elastic beam 2, to which the nib 1 is attached. The reflectedpart of laser beam 22 falls on a mirror 23, which directs the laser beam22 through a beam splitter 32 and onto a bisected photodiode 24, which,as compared to the frequency of the ultrasound, is slow.

The arrangement of mirror 23 and photodiode 24 is such that the laserbeam 22 falls at a specific interactive force between nib and specimen11, with nib 1 in center position, in the center between the firstelement 25 and second element 26 of the bisected photodiode 24.

The photoelectric voltages of the first element 25 and second element 26act by way of output lines 27, 28 on a standardizing amplifier 29, whichstandardizes and amplifies the difference between the photoelectricvoltages of elements 25, 26 to their sum value. The output signal of thestandardizing amplifier 29 acts via a line 30 on an input of ananalog-digital converter 31 that digitizes the voltages.

The beam splitter 32 divides the laser beam 22 incident from mirror 23at a ratio of about 1:1. The arrangement of the beam splitter 32 is suchthat the laser beam 33 reflected on it falls on a mirror 34 which, inturn, directs the reflected laser beam 33 through a collimating lens 35and past a razor blade 36 onto a monocell photodiode 37 whose bandwidthat least equals the frequency of the ultrasound.

The beam splitter 32, mirror 34, collimating lens 35 and razor blade 36are so arranged that, with centered alignment of the laser beam 22passing through the beam splitter 32 on the bisected photodiode 24, therazor blade 36 baffles the reflected laser beam 33 substantially atcenter symmetry. The nonbaffled share of the reflected laser beam 33falls on the monocell photodiode 37. The optical path lengths in thisstructure are in the centimeter range.

The monocell photodiode 37 is via line 38 wired to a load resistor of 50ohms and to a protective resistor, which are illustrated as a resistancecircuit 39. The monocell photodiode 37 is in the present embodiment asilicon pin diode with a rise time of about 1 nanosecond. But otherlight-sensitive detectors with a bandwidth greater than the frequency ofthe ultrasound coupled in, such as avalanche diodes, are suitable aswell. The output signal of the resistance circuit 39 proceeds via a line40 to the input of an amplifier 41 with approximately 60 decibelamplification. The output signal of amplifier 41 proceeds via a line 42to a voltage input of the transceiver 19.

The data output of transceiver 19 connects via line 43 to a second inputof the analog-digital converter 31 capable of digitizing the voltagefrom the transceiver 19. The data digitized by the analog-digitalconverter 31 can be fed, via a line 44, to the control and datacapturing device 17. The digitized output signals from the standardizingamplifier 29 and transceiver 19 can be stored in a memory of the controland data capturing device 17 in contingence on the position of specimen11. An output unit 76, for instance a monitor, is provided to output thecontent of the memory of the control and data capturing device 17 in theform of a color or gray value coding.

FIG. 2 shows in a block diagram the inner structure of transceiver 19.The signal of amplifier 41, in FIG. 1, prevailing on the line 42 ispresent on a channel 45 of an oscilloscope 46 and on the signal input 47of a time gate integrator 48.

A supersonic transmitter 57 emits a brief pulse with about 15nanoseconds rise time, via line 18, to the transmitting head 9. A pulsegenerator 49 receives at the same time a start pulse via a line 56. Inresponse, the pulse generator 49 emits via a line 51 a rectangular pulseof adjustable duration to a delay circuit 52 that is capable ofretarding the rectangular pulse in an adjustable fashion relative to thestarting pulse. The retarded rectangular pulse can be fed via delay-line53, for one, to a second channel 54 of oscilloscope 46 and, for another,to the time gate input 55 of the time gate integrator 48. The timedeflection of oscilloscope 46 is triggered by the pulse generator 49 vialine 75. Oscilloscope 46 allows controlling the position of the retardedrectangular pulse from the delay circuit 52 as regards the signalprevailing on channel 45.

The average amplitude prevailing on the signal input 47 can be set withthe time gate integrator 48 while the high level of the retardedrectangular pulse from the pulse generator 49 is present on the timegate input 55. The average amplitude value can be fed via line 43 to theanalog-digital converter 31 of FIG. 1 for digitizing.

The repetition rate of the supersonic transmitter 57 is such that thesupersonic amplitude values will be updated at least once in eachposition of the specimen 11, before the control and data capturingdevice 17 reads a new amplitude value. In the present embodiment, thesupersonic repetition rate is about 1 kHz, so that up to a read-infrequency of about 500 Hz there is assurance that at least onesupersonic pulse will be transmitted at each position of the specimen11.

As previously mentioned, the mean interactive force between nib 1 andspecimen 11 must be kept constant during measurement. This can beaccomplished in that, in a control circuit, the part of laser beam 22transmitted through the beam splitter 32 is held centered, via controland data capturing device 17 and by appropriate feedback loop adjustmentof the z-electrode 8, between the first element 25 and second element 26of the bisected photodiode 24. At positive or negative changes of theoutput level of standardizing amplifier 29, the voltage on thepiezoelectric control line 14 to the z-electrode 8 allows adjustmentsuch that the-output voltage of the standardizing amplifier 29approaches neutral value. In contingence on the x- and y-positions ofspecimen 11, the control voltage for the z-electrode 8 provides an imageof the topography of specimen 11.

The frequency of the ultrasound is adjustable between about 5 MHz andabout 100 MHz, ranges thus far above the resonant frequency of severalKHz of the elastic beam 2 with the nib 1 as used in the presentembodiment.

Concerned with the ultrasound coupled in, in the present embodiment, arelongitudinal waves, so that the surface of specimen 11 performs normalvibrations. These normal vibrations result in a high-frequencydeflection of the elastic beam 2, which, however, escape detection dueto the insufficient bandwidth of the slow, bisected photodiode 24, butresult in the monocell photodiode 37 with a bandwidth in the megahertzrange, due to baffling part of the reflected laser beam 33 with therazor blade 36, in a measuring signal.

The delay circuit 52 in this embodiment is adjusted such that theaverage amplitude of the first high-frequency deflection of elastic beam2, in response to the surface vibration of specimen 11, can be selectedin the time gate integrator 48. Upon storing a value for the deflectionof nib 1, the specimen 11 may be positioned on the x-electrode 6 andy-electrode 7 at the next measuring point, by means of appropriateoutput signals of the control and data capturing device 17, followed bythe next ultrasound measuring cycle.

During rasterization of the surface of specimen 11 across thepredetermined measuring area, the control voltages of the z-electrode 8for imaging the topography of the surface of specimen 11, matched tolocation, are displayed on the output unit 76, along with alocation-matched representation of the supersonically induced deflectionof nib 1 by way of the respective measuring values. A customary grayvalue or color coding is preferably chosen for this representation. Inthis fashion, both an image of the topography of the surface of specimen11 and an image of the amplitudes of the supersonic waves at the surfaceof specimen 11, matching the elasticity properties of the specimensurface, can be generated simultaneously in metrologically independentfashion.

For metrological scanning of other time ranges of the emitted ultrasoundit is possible to timewise vary the time gate by means of the delaycircuit 52. This is necessary especially when the intermediate body 10has been changed along with the passage time.

FIG. 3 illustrates schematically a further embodiment of an acousticmicroscope where the components corresponding to those of FIG. 1 arereferenced identically and not illustrated hereafter in any detail. Inthe embodiment according to FIG. 3, the laser beam 33 reflected by thebeam splitter 32 is coupled into a heterodyne travel time interferometer58. The reflected laser beam 33 falls on an input beam splitter 59 whichcouples approximately one-half of the incident intensity into the longarm 60 of the heterodyne travel time interferometer 58.

The long arm 60 contains two mirrors 61, 62 which direct the light inthe long arm 60 through a Bragg cell 63 operating on the basis of theacousto-optic effect onto an output beam splitter 64. The arrangement ofthe mirrors 61, 62 and of the Brag cell 63 is such thatfrequency-shifted output light of first-order refraction, of the Braggcell 63, falls on the output beam splitter 64. The Brag cell 63 connectsvia a line 65 to an activation circuit 66, by means of which the amountof frequency shift has in the present embodiment been set to about 80MHz.

The short arm 67 of the heterodyne travel time interferometer 58 isformed by the share of the laser beam 33 transmitted through the inputbeam splitter 59, of about one-half its intensity. The light pencils ofthe long arm 60 and short arm 67 are collinearly superimposed in theinterference beams 68, 69 through the output beam splitter 64.

The first interference beam 68 falls on a first interference photodiode70, the output signal of which acts via line 71 on a first input of adifference demodulation amplifier 72. The second interference beam 69falls on a second interference photodiode 73, the output signal of whichacts via line 74 on a second input of the difference demodulationamplifier 72.

Because of the difference in travel time of the light between the shortarm 67 and the long arm 60, the output signals of the interferencephotodiodes 70, 73 contain in customary fashion phase modulations thatare proportional to the deflection differences of the nib 1. The traveldifference between arms 60, 67 is preferably so chosen that thesensitivity of the interferometer to the incident supersonic frequencyis maximal, meaning that the difference in arm length equals about thespeed of light divided by twice the supersonic frequency multiplied bythe index of refraction.

The difference demodulation amplifier 72 is capable of forming thedifference between the output signal of the first interferencephotodiode 70 and the second interference photodiode 73; this differencesignal allows decoupling from low-frequency interferences, by thefrequency shift in the Bragg cell 63 with subsequent demodulation. Thismakes it possible to even out, e.g., intensity fluctuations in thereflected beam 33 or low-frequency mechanical disturbances. Moreover,the difference signal is suited for amplification in the differencedemodulation amplifier 72 and acts by way of line 42 on the time gateintegrator 48 of transceiver 19.

In a modified embodiment, the heterodyne travel time interferometer 58has been substituted by an actively stabilized fiber-optic travel timeinterferometer. The stabilized interferometer features in its long arm aphase shifter which in a control circuit connects via a line to thedifference amplifier, so that changes of the optical path length can beevened out. Coupling in and out, the same as in the embodimentillustrated in FIG. 3, is effected using the heterodyne travel timeinterferometer.

FIG. 4 illustrates a further embodiment of the invention where thecomponents corresponding to those in FIG. 1 and 3 are referencedidentically and will not be described in any detail hereafter. In theembodiment relative to FIG. 4, the deflections of nib 1 can be detectedby means of a capacitive detector device. Said device has a measuringcapacity which is formed by the metallically coated back of the elasticbeam 2 and a stylus type opposite electrode 77, the point of which isaligned on the elastic beam 2. The latter is attached to an electricallyconducting holder 78 that connects to a ground 79.

The opposite electrode 77 is mechanically attached to a piezoelectricelement 80 and electrically decoupled from it. Said piezoelectricelement 80 is mounted on a mechanical positioning table 81 which, inturn, is mounted on the holder 4. The piezoelectric element 80 is actedupon by a voltage from an adjustable power supply 83, by way of line 82.The opposite electrode 77 allows positioning at a distance above theelastic beam 2, with rough positioning through mechanical adjustment ofthe positioning table 81, and precision positioning by way of modifyingthe supply voltage of the piezoelectric element 80.

The opposite electrode 77 connects electrically via a line 84 to acapacity measuring circuit 85. The measuring capacity is integrated inan oscillating circuit of the capacitive measuring circuit 85, which isstimulated by an oscillator vibrating at about 915 MHz. The oscillatorfrequency is so selected that it is in the steepest area of the flank ofthe resonance curve of the oscillating circuit. Changes in measuringcapacity, by deflections of nib 1 and thus of elastic beam 2, result ina change of the resonant frequency of the oscillating circuit, and thusof the amplitude stimulated by the oscillator. These amplitude changesare splittable in a low-frequency and a high-frequency share by alow-pass filter 86 and a high-pass filter 87.

The low-pass filter 86 has a passband up to about 20 kHz, while thepassband of the high-pass filter 87 starts at about one-half thesupersonic frequency value. This makes it possible to tap the slowdeflections of the nib 1, produced by topography changes of the surfaceof specimen 11, at a low-frequency output 88 while tapping thehigh-frequency supersonically induced deflections of the nib 1 at ahigh-frequency output 89.

The signal prevailing on the low-frequency output 88 can be passed vialine 30 to the analog-digital converter 31 and serves as a controlsignal for keeping the average distance between the surface of specimen11 and nib 1 constant. The signal prevailing on the high-frequencyoutput 89 is suited for coupling into the transceiver 19 via line 42.The further data processing proceeds as described in conjunction withFIG. 1, 2, and 3.

We claim:
 1. An acoustic microscope for the examination of a specimen,with a tip fastened to a cantilever, said tip being in thesurface-near-zone of the specimen, said microscope comprising anultrasonic transducer, a scanning device for positioning the specimenrelative to the tip, and with a device for control and data acquisition,whereina) the ultrasonic transducer includes a transmitter with meansfor providing ultrasound coupled to the specimen, b) the ultrasound hasa frequency which exceeds the lowest resonance frequency of thecantilever with the tip fixed to the cantilever, c) the cantilever, aclamp with a resonance frequency lower than the ultrasonic frequency,the scanning device, the transmitter, and the specimen are mechanicallyrigidly coupled to one another, and d) with a detection unit installedwhich measures the deflections of the tip caused by the topography ofthe specimen, the deflections being of lower frequency in relation tothe ultrasonic frequency in a first detection signal, and the detectionunit also measuring high-frequency displacements of the tip caused bythe ultrasound in the specimen in a second detection signal, and meansfor maintaining a constant distance between the tip and the surface ofthe specimen by utilizing a feedback loop in the control and dataacquisition device, the feedback loop serving to average the firstdetection signal using a time large as compared to the period of theultrasonic frequency and with a scanning rate small as compared to theultrasonic frequency, anda light beam reflected by the cantilever can becoupled, via optical devices for beam deflection, into a detection unitwhich comprises a first optical detection unit connected with thefeedback loop and a second optical detection unit for generation of thesecond detection signal, said second detection unit contains aheterodyne time-of-flight interferometer whose long reference armcontains a frequency shifter in order to shift the frequency of thelight-beam in the long reference arm.
 2. An acoustic microscope, asdefined in claim 1, characterized in that the interfering beams of theheterodyne time-of-flight interferometer are directed each onto alight-sensitive detector whose output signals are subtracted from oneanother in a difference demodulation amplifier, and demodulated from thefrequency of the frequency shifter and amplified.